Re: (ET) Small E20 fire: What is it with MOVs?

[RJ Kanary <rjkanary consolidated net>]

    With the field weakened would not the temporary output of this 
motor turned generator be reduced ? Unless the cruise button was held 
in, mashing the throttle would result in field weakening, would it not 
?  Weakened field, not much power generated.

     And BTW holding the cruise button in while mashing the throttle 
will result in dramatic regenerative braking, (No field weakening.) to 
the extent that the Power Use meter will be pinned against the left hand 
stop.{ Still without smoke escaping from the MOV in the times that I've 
done it. :)   }

RJ

Rhett George wrote:
> > Date: Sun, 13 Mar 2011 17:27:54 -0400
> > From: Chris Zach <cz alembic crystel com>
> > To: elec-trak cosmos phy tufts edu
> > Subject: (ET) Small E20 fire: What is it with MOVs?
> > Message-ID: <4D7D36DA 8020909 alembic crystel com>
> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> >
> > So I was using the E20 pretty heavily today in the yard, mostly to tow 
> > the trailer full of dirt. Once done I drove it up to the top of the 
> > driveway to get the paper, then let it coast in L down to check the brakes.
> >
> > Brakes stink like normal but can still stop the tractor (new brake kit 
> > going on after spring work). Fired it up again, let it build up speed, 
> > then engaged the motor to slow it down.
> >
> > *Foom* Big cloud of white smoke from under hood. That's always bad. 
> > Motor still had power, but I stopped the tractor and hit the main 
> > disconnect. Smoke continued to pour out for a moment, then it lightened up.
> >
> > Hm. Opened the main panel, and looked at card 4. Everything was intact 
> > (I have bigger diodes on that) however on the resistor card there was a 
> > smoking disc where the MOV once was. Looks like it blew up.
> >
> > So what is it with E20's and MOVs? I know they're there to catch the 
> > back emf from the motor or the spikes or whatever but they just seem to 
> > explode every year. Can one get bigger/higher capacity MOVs? Can one 
> > parallel them? Do something else?
> >
> > Chris
> >
> >     
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>  - Greetings to Chris and the others -
>
> Gathering a few points from the first two paragraphs, the tractor with 
> Chris
> aboard was coasting downhill in L and going faster and faster.  The motor 
> was engaged and "foom" was produced.  
>
> It is a guess but since the transmission is not synchromeshed and there is
> no clutch, the motor was engaged by stepping on the GO pedal.  If the 
> tractor
> was going twice as fast as it would with the cruise control ON, then the 
> twice-as-fast spinning armature wouold be generating about 72 volts.  Power 
> dissipated (briefly) in the MOV which is not a resistor increases at an 
> exponential rate greater than V squared.  The resulting heat escaped, taking
> pieces of the MOV with it.  Like Humpty-Dumpty it cannot be put back 
> together
> again.  Sigh.
>
> And cheers,
>
> Rhett
>
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